1. Technical Field of the Invention
The present invention relates to catalytic naphtha reformers and catalytic reforming processes. More particularly, this invention relates to a method and apparatus to take advantage of thermodynamic and chemical equilibrium parameters to increase efficiency of the processes in producing larger quantities of octane enhancing components and reducing the amount of gas formed thereby lowering operating cost.
2. Description of the Prior Art
Catalytic reforming is generally used to reform low octane naphthas into high octane gasoline blending components referred to as reformates. Numerous reactions such as cracking, polymerization, dehydrogenation, and isomerization occur simultaneously during reforming. Depending upon the properties of the naphtha feedstock and catalysts used, reformates can be produced with high concentrations of such constituents as benzene, toluene, xylene, and other aromatic compounds that are useful in gasoline blending and petrochemical processing.
Generally, the feedstock to a reformer is a naphtha stream, which includes three types of organic chemical compounds with carbon numbers in the range of five to ten. These compounds are classified primarily as paraffins, naphthenes, and aromatics. Each of these chemical compounds reacts differently in the presence of the typical dual functionality reforming catalyst. One of the functions is the rearrangement or isomerization reaction performed by the acidic site while the second is the hydrogenation/dehydrogenation reaction performed by the metallic site.
The goal of most naphtha catalytic reforming processes is to form aromatic compounds with high octane rating. The napthenes in the naphtha feed streams react very quickly to form aromatic compounds. Paraffins, on the other hand, are very unreactive and require higher temperatures to be converted to aromatic compounds. The aromatic compounds essentially undergo very little reaction in the normal situation. However, these aromatic compounds can undergo cracking reactions in environments with temperatures in the range of about 960° F. to 980° F. In particular, the rate of formation for toluene and higher carbon atoms compounds appear to level off at around 970° F. to about 980° F., while that of benzene continues to increase in the same temperature range.
Naphthenes react very quickly and convert to aromatics to the extent allowed by equilibrium considerations by the time the reacting medium exits the first of three or four reforming reactors in series in a typical reforming process. This, in essence, means that all naphthenes are basically depleted in the first reactor. After the naphthenes convert to aromatics then it becomes imperative to convert the paraffins, which is much harder to do since there are more steps required to convert a typical paraffin to an aromatic compound. For example, hexane has to be converted to cyclohexane then it has to undergo a dehydrogenation step to become benzene. Energy and favorable equilibrium conditions are required for these extra steps to occur. The energy required is generally supplied by external means in the form of heat.
Since these reforming reactions are generally endothermic in nature, the feed to each of the several reactors in series in the reforming train is heated to reaction temperature in external heaters to the reactors.
The competition between the reactions to form aromatics and cracking or dealkylation reactions undergone by the aromatics at higher temperatures cause most catalytic processes to be inefficient. In such environments, aromatic compounds are being formed and depleted concurrently. These competing reactions tend to discourage using higher temperatures in reactors in the first place because the higher operating temperatures tend to cause some of the aromatic compounds to undergo cracking or de-alkylation reactions, which result in the formation of undesirable hydrocarbons with carbon numbers less than five. At the same time, higher temperatures are required to cause the reactions of paraffins to aromatics. It is desirable in reforming reactions to maximize formation of aromatic compounds with carbon numbers in the range of about six to about ten while, at the same time, minimize the formation of gaseous hydrocarbons with carbon numbers less than five.
Others have attempted to increase the amount of aromatic compounds that are produced in reforming processes. One such example can be found in U.S. Pat. No. 4,401,554 issued to Choi et al. (hereinafter “Choi”). In Choi, a naphtha feed stream is separated into two fractions prior to being sent to a reformer reactor train. The first fraction, which contains the heavy fraction, is sent through at least three catalyst zones and the second fraction, which contains the light fraction, is only sent through to the last catalyst zones. The heavy fraction passes through the entire sequence of catalytic reactors undergoing severe reforming conditions in terms of temperature and time while the light fraction is processed only in the last one or two reactors. These severe conditions lead to dealkylation reactions of C7 and heavier aromatic compounds particularly at high temperatures. This tends to reduce the total amount or volume of reformate composed of C5 and heavier and impacts the economics of the process.
In U.S. Pat. No. 5,672,265 issued to Schmidt et al, this patent discloses a reforming process for full range naphtha. The effluent of the last reactor in the reformer train is separated into fractions. The light fraction is composed of hydrogen and hydrocarbons lighter than C5 and the heavier fraction is composed of the reformate for use in gasoline blending. The reformate stream is further treated through an extractive distillation column or beds of molecular sieves to extract paraffinic compounds in the molecular weight range of C6-C8. These paraffinic compounds are fed to a reformer-type reactor containing an aromatization catalyst, as opposed to a reforming catalyst. The inclusion of the extractive distillation process is cost prohibitive, particularly when coupled with the reforming-type step.
Another process for increasing the amount of aromatic compounds produced in a reforming process is described in U.S. Pat. No. 4,950,385 issued to Sivasanker et al. (hereinafter “Sivasanker”). In Sivasanker, two different catalysts are used in two catalyst zones within the reactor train. The reformate stream from the second catalyst zone is split into two fractions with the high pressure fraction, hydrogen and hydrocarbons with carbon numbers less than three primarily being recycled back to the first catalyst zone containing a conventional reforming catalyst and the low pressure fraction being recycled back to the catalyst zone containing an acidic reforming catalyst having a crystalline iron silicate. However, to use separate reactor trains to hold each of the two such catalyst can be relatively expensive to operate when compared with the use of single conventional reforming catalysts.
A need exists for a more economical and efficient method of increasing the amount of aromatics that are produced from a hydrocarbon stream during catalytic reforming. It would be advantageous to provide a method that makes it easier to convert the paraffins to aromatics while simultaneously reducing the cracking or dealkylation tendency of the aromatic compounds. A process apparatus to increase the amount of aromatic compounds produced from a hydrocarbon stream that uses smaller reactors than conventional reforming processes would be advantageous from the investment and operating cost perspective. Additionally, it would be advantageous to add the modified catalytic reforming process to an existing catalytic reforming process.